[OSM-talk] [tagging] absent tags (was: noname streets)
vincent at zweije.nl.eu.org
Mon Jun 9 13:01:46 BST 2008
On Mon, Jun 09, 2008 at 11:58:28AM +0100, Richard Fairhurst wrote:
|| Dave Stubbs wrote:
|| > The good point about name=__none__ is that I can bet large amounts of
|| > money that no street is actually named "__none__" -- the bad points
|| > are that renderers that don't know about it are going to write it in
|| > the street name
|| So maybe named=no (or unnamed=yes)?
The problem is really more general.
You want to be able to say (a): "tag T is absent", as opposed to (b)
"nobody has bothered to enter tag T yet".
For distinguishing these two, we need to find some way to explicitly
specify (a), because (b) has to be modeled by the tag just being not there
(think about it).
I can see several ways, in personally increasing preference:
1. define a special value for the tag (e.g. "" or "__none__")
disadvantage: takes away a technially valid value, so may not be
generalizable to tags other than "name".
2. add another (meta) tag specifying which tags are undefined:
disadvantage: this is a multi value tag, which complicates the
model. Witness the gratuitous potlach value-combining with semicolons.
3. specify a derived tag, as with localised names:
disadvantage: might accumulate to a lot of extra tags in the database,
but it only needs to be added if there really is uncertainty about
So... how about doing T:absent=true? The interpretation of which being:
tag T is not there in the real world, as opposed to only not there in
Vincent Zweije <zweije at xs4all.nl> | "If you're flamed in a group you
<http://www.xs4all.nl/~zweije/> | don't read, does anybody get burnt?"
[Xhost should be taken out and shot] | -- Paul Tomblin on a.s.r.
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